1. Field of the Invention
This invention relates to an electrically conductive side-by-side biconstituent fiber consisting essentially of relatively nonconductive synthetic thermoplastic fiber-forming polymer and electrically conductive material, such as carbon black.
2. Description of the Prior Art
The annoying effects caused by the buildup of static electricity on the surface of fabrics constructed from nonconductive fibers such as polyamide fibers is generally known. Several approaches have been taken in the past by the textile industry to significantly reduce such effects. One approach which has been quite successful in this regard is to intermingle electrically conductive carbon black-containing fibers with the nonconductive fibers in such fabrics. The carbon black serves to dissipate the static electricity which would otherwise build up on the fabric surface. Several techniques have been used to incorporate carbon black into nonconductive fibers.
One technique, described in U.S. Pat. No. 3,803,453, consists of preparing a sheath-core filament in which carbon black is dispersed in the core component. This technique, however, has not been entirely effective in reducing the static electricity below the 3500 volt level of normal human sensitivity.
Another technique, described in U.S. Pat. No. 3,823,035, consists of penetrating the surface of drawn fiber with carbon black. The penetration is accomplished by applying to the drawn fiber a dispersion consisting of carbon black in a liquid which is a solvent for the fiber. The liquid is removed from the fiber after the desired degree of penetration is obtained but before structural integrity of the fiber is destroyed. The application of the dispersion and subsequent removal of the liquid from the fiber requires extra processing steps, is time consuming and involves the handling of hazardous materials. Since the fibers are drawn prior to applying the dispersion, the resulting fibers cannot be easily combined or blended with nonconductive staple fibers by conventional methods. Also, during normal usage of the fibers, the carbon black tends to flake from the fibers.
Yet another approach for incorporating electrically-conductive carbon black into fibers is described in U.S. Pat. No. 3,969,559. This approach consists of providing a side-by-side biconstituent fiber composed of a nonconductive constituent (e.g. polyamide) which wraps around (i.e. partially encapsulates) a conductive constituent. The conductive constituent constitutes from 1 to 30% by volume of the fiber and is composed of a nonconductive polymer (e.g. polyamide) containing electrically-conductive carbon black dispersed therein. The carbon black is present in the conductive constituent in an amount sufficient to provide a fiber having an electrical resistance of less than 1 .times. 10.sup.10 ohms per centimeter at a direct current potential of 0.1 volt when measured at 20% relative humidity and 21.degree. C. The conductive constituent extends longitudinally along the surface of the fiber and inwardly from the surface such that its width is less than the length of the interface of the two constituents. The interface is convex (conductive constituent)/concave (nonconductive constituent). Textile articles containing only a small amount of these fibers are rendered static resistant and retain such property even after prolonged use.
Unfortunately, from the standpoint of commercial operations, considerable process control must be exercised in order to provide a fiber having the configuration of the constituents described in U.S. Pat. No. 3,969,559. In general, this configuration is achieved by maintaining proper control of the melt viscosity of each constituent during melt spinning which severely limits the raw material specifications and permissible spinning conditions. For example, the starting materials (each constituent in flake form) must be extremely dry. If the moisture level of the conductive constituent is greater than about 0.05%, the configuration of constituents is no longer obtainable. To provide and control the moisture level of the constituents at 0.05% or less is troublesome and requires expensive equipment. Also, each constituent must be maintained at a different temperature during the spinning operation so as to keep each constituent at the proper melt viscosity. As a result, expensive and complex twin spinning blocks with twin heating medium supplies and twin transfer lines, each having separate temperature and pressure controls, are required. It has also been found that conventional additives (e.g. stabilizers and extenders) cannot be incorporated into the conductive constituent without adversely affecting the melt viscosity of the conductive constituent.
An object of the invention is to provide a conductive fiber which is easier and less costly to produce than the fiber described in U.S. Pat. No. 3,969,559.
Another object of the present invention is to provide a conductive fiber having an electrical resistance equal to or less than that of the fiber described in U.S. Pat. No. 3,969,559.